1. Field of the Invention
The present invention relates to a liquid crystal display device having a wide viewing angle characteristic suitable for use in a liquid crystal display apparatus having a flat display such as a personal digital assistant, a personal computer, a word processor, an amusement apparatus, an educational apparatus, a television set, or the like, which may be viewed by a large number of people at the same time, and in a display board, a window, a door, a wall, or the like, utilizing a shutter effect.
2. Description of the Related Art
Various types of liquid crystal display devices have been used in the art, including TN (Twisted Nematic) and STN (Super Twisted Nematic) type liquid crystal display devices in which a voltage is applied through the liquid crystal layer to untwist the twisted liquid crystal molecules so as to obtain brightness/darkness, and other types of liquid crystal display devices in which a voltage is applied to change the orientation of the liquid crystal molecules from the initial orientation to another orientation so as to impart a change in birefringence to incident light, thereby obtaining brightness/darkness or color display.
However, these conventional liquid crystal display devices typically have a narrow viewing angle. Therefore, various techniques have been developed in order to increase the viewing angle.
Methods for increasing the viewing angle of a liquid crystal display device include a method in which the liquid crystal molecules are moved in a direction substantially parallel to the surface of the substrate, and a method in which each pixel is divided into a plurality of regions having different orientations with the movement of the liquid crystal molecules being perpendicular to the surface of the substrate. A typical example of the former type of method is an IPS (In-Plane-Switching) mode. As examples of the latter type of method, various liquid crystal display modes have been proposed in the art, including a wide viewing angle liquid crystal display mode in which Np (Nematic Positive) type liquid crystal molecules are horizontally oriented in axial symmetry (Japanese Laid-Open Publication No. 7-120728), another wide viewing angle liquid crystal display mode in which vertically aligned Nn (Nematic Negative) type liquid crystal molecules are horizontally oriented in axial symmetry upon application of a voltage (Japanese Laid-Open Publication No. 10-186330), another wide viewing angle liquid crystal display mode in which vertically aligned Nn (Nematic Negative) type liquid crystal molecules are oriented in a time division manner by controlling an electric field applied therethrough (Japanese Laid-Open Publication No. 7-64089), and another wide viewing angle liquid crystal display mode in which Np (Nematic Positive) type liquid crystal molecules are horizontally oriented while generally dividing each pixel into four regions (AM-LCD ""96, P. 185 (1996)).
Among others listed above, Japanese Laid-Open Publication No. 7-120728 discloses a wide viewing angle liquid crystal display mode called xe2x80x9caxially symmetric aligned microcell mode (or Np-ASM mode)xe2x80x9d, where Np type liquid crystal molecules are oriented in axial symmetry in each pixel. In a display device of this mode, each pixel is divided into a plurality of liquid crystal regions each of which is substantially surrounded by polymer walls formed by phase separation from a mixture of a liquid crystal material and a photocurable resin. The liquid crystal molecules in each pixel are oriented in axial symmetry. According to this technique, a normally white display is produced by applying a voltage through liquid crystal molecules in an axially symmetric orientation so as to orient the liquid crystal molecules perpendicular to the substrate. The technique disclosed in Japanese Laid-Open Publication No. 10-186330 employs an Nn-ASM display mode with an Nn type liquid crystal material, and produces a normally black display. The liquid crystal molecules are oriented perpendicular to the substrate in the absence of an applied voltage, and when a saturation voltage is applied, the liquid crystal molecules are oriented in the respective liquid crystal regions in a pixel each of which is substantially surrounded by polymer walls so that the liquid crystal molecules are oriented in axial symmetry for each pixel. Japanese Laid-Open Publication No. 7-311383 discloses a technique in which the liquid crystal orientation is divided into four directions by providing an orientation controlling slope using an orientation controlling layer on each substrate.
Japanese Laid-Open Publication No. 7-64089 discloses a technique in which an orientation controlling electrode is provided between pixels for applying a driving voltage which is higher or lower than that of all the other transparent electrodes. An orientation controlling window is provided in the transparent electrode in the form of a gap in the electrode. This structure allows for adjustment of the electric field through the liquid crystal layer, thereby controlling the orientation of the liquid crystal molecules. The gaps in the transparent electrode are patterned in a xe2x80x9cxxe2x80x9d shape on one of a pair of substrates. On the other substrate, the orientation controlling electrodes are provided in a lattice pattern. By superimposing these patterns on each other, the electric field applied through the liquid crystal layer is bent, thereby realizing a xe2x80x9c4-division ECBxe2x80x9d where the liquid crystal orientation is divided into four directions. However, when these patterns are used, a disclination line occurs when the voltage is turned ON/OFF, thereby affecting the voltage response characteristics and thus deteriorating the display quality. Moreover, in this conventional technique, the liquid crystal material does not contain a chiral agent, and the color shift has a substantial cell thickness dependency in a 4-division mode, thereby requiring a high precision in controlling the cell thickness. Therefore, it is very difficult to obtain a uniform display quality for a large area display device.
In the liquid crystal display device of the Np-ASM mode, which employs a normally white mode, the light-blocking portion of the BM (black matrix) needs to have a large area in order to properly prevent light leakage when the voltage is OFF. Moreover, the production of an ASM mode display device has been relatively difficult because it involves a phase separation process, which requires a precise temperature control. Furthermore, both the Np-ASM mode and the Nn-ASM mode commonly suffer from the following problems:
{circle around (1)} The production cost is high due to the use of a photocurable resin.
{circle around (2)} The number of production steps is increased and the production process is complicated, thereby increasing the production cost because of the step of irradiating the liquid crystal molecules with UV light with the liquid crystal molecules being orientated in axial symmetry by an application of a voltage so as to cure the photocurable resin, thereby providing an orientation stabilizing layer, and the step of patterning the polymer walls by photolithography. Moreover, according to this method, dust is likely to attach to the substrate, thereby increasing the defect rate due to a defective liquid crystal orientation.
{circle around (3)} When fixing the orientation of the liquid crystal molecules, the UV light used to irradiate the liquid crystal molecules decomposes the liquid crystal material, the polymer wall material and the orientation film material, thereby lowering the voltage retention and thus lowering the reliability of the display (e.g., an image burn phenomenon may occur).
When the orientation stabilizing layer is not provided, the tilt direction of the liquid crystal molecules will not be well stabilized. Then, the response speed of the display device is reduced, whereby a stable ASM orientation may not be obtained upon driving the device, resulting in non-uniformity in displayed images.
According to one aspect of this invention, a liquid crystal display device includes a pair of electrode substrates with a liquid crystal layer being interposed therebetween. No-electrode areas, where no electrode exists, are provided in a discrete pattern partially on at least one of the pair of electrode substrates. A pixel, which is a minimum unit of display, is defined by being surrounded by a plurality of the no-electrode areas. Liquid crystal molecules are oriented in an axially symmetric orientation in each pixel when a voltage is applied between the pair of electrode substrates.
In one embodiment of the invention, each pixel includes an area in which the liquid crystal molecules are maintained in a vertical alignment in the presence of an applied voltage.
In one embodiment of the invention, a threshold voltage in voltage-transmission characteristics for the area in which the liquid crystal molecules are maintained in a vertical alignment in the presence of an applied voltage is equal to or greater than 1.5 times that for other areas, in which the liquid crystal molecules do not retain the vertical alignment in the presence of an applied voltage, and a saturation voltage in voltage-transmission characteristics for the area in which the liquid crystal molecules are maintained in a vertical alignment in the presence of an applied voltage is equal to or greater than 1.5 times that for the other areas, in which the liquid crystal molecules do not retain the vertical alignment in the presence of an applied voltage.
In one embodiment of the invention, the no-electrode areas are provided in a discrete and regular manner in each pixel. The pixel has a square shape or a rectangular shape. An axis of the axially symmetric orientation exists in the pixel.
In one embodiment of the invention, the no-electrode area has a square shape or a circular shape. Expressions 1 and 2 below are satisfied, wherein D (xcexcm) denotes a distance between two adjacent no-electrode areas, ds (xcexcm) denotes a length of each side or a diameter of the no-electrode area, and y=D/ds.
Expression 1: 20xe2x89xa6dsxe2x89xa650
Expression 2: 0.1xe2x89xa6yxe2x89xa6xe2x88x920.025ds+2.25
Each pixel defined by the no-electrode areas has a square shape or rectangular shape. If the pixel has a square shape, a size of the pixel is 20 xcexcmxc3x9720 xcexcm to 200 xcexcmxc3x97200 xcexcm. If the pixel has a rectangular shape, a length of each longer side of the pixel is 20 xcexcm to 200 xcexcm and an aspect ratio of the pixel (the ratio between the length and the width: xe2x80x9crxe2x80x9d) is 1 less than r less than 2.
In one embodiment of the invention, each no-electrode area has a rectangular shape. Expressions 3 and 4 are satisfied, wherein P (xcexcm) denotes xc2xd of a distance between two adjacent no-electrode areas, ps (xcexcm) denotes a length of the longer side of the no-electrode area, and Y=P/ps.
Expression 3: 60xe2x89xa6psxe2x89xa6120
Expression 4: xe2x88x9210xe2x88x926ps3+0.0004ps2xe2x88x920.0578ps+2.325xe2x89xa6Yxe2x89xa6xe2x88x9210xe2x88x926ps3+0.0004ps2xe2x88x920.0578ps+3.325
Each pixel defined by the no-electrode areas has a square shape or a rectangular shape. If the pixel has a square shape, a size of the pixel is 20 xcexcmxc3x9720 xcexcm to 200 xcexcmxc3x97200 xcexcm. If the pixel has a rectangular shape, a length of each longer side of the pixel is 20 xcexcm to 200 xcexcm and an aspect ratio of the pixel (the ratio between the length and the width: xe2x80x9crxe2x80x9d) is 1 less than r less than 2.
In one embodiment of the invention, the liquid crystal layer includes a nematic liquid crystal material. the liquid crystal molecules in the nematic liquid crystal material are oriented substantially vertical to a surface of each of the pair of electrode substrates in a black display and are oriented in an axially symmetric orientation in a white display.
In one embodiment of the invention, the liquid crystal layer includes a vertical alignment layer and a nematic liquid crystal material having a negative dielectric anisotropy. The liquid crystal molecules of the nematic liquid crystal material are oriented substantially vertical to a surface of each of the pair of electrode substrates in the absence of an applied voltage.
In one embodiment of the invention, the device further includes: a liquid crystal cell including the pair of electrode substrates and the liquid crystal layer interposed between the pair of electrode substrates: a pair of polarizing plates interposing the liquid crystal cell therebetween; and a phase difference compensator provided between the liquid crystal cell and at least one of the pair of polarizing plates. Each of the polarizing plates and the phase difference compensator has three refractive indices nx, ny and nz along x, y and z axis directions, respectively, which are orthogonal to one another. The refractive indices nx and ny are primary refractive indices along a plane of the liquid crystal cell, and refractive index nz is a primary refractive index along a thickness direction of the liquid crystal cell. The maximum refractive index nx axis is perpendicular to an absorption axis of one of the polarizing plate which is provided on one side of the cell closer to the viewer. The relationship nz less than ny less than nx holds between the refractive indices nz, ny and nx.
In one embodiment of the invention, a cell thickness keeping member for keeping a thickness of the liquid crystal layer is provided outside the pixel or in the no-electrode area.
In one embodiment of the invention, the device is applied to a plasma-addressed liquid crystal display device (PALC), a thin film transistor (TFT), or a diode.
According to another aspect of this invention, there is provided a method for producing a liquid crystal display device. The device includes a pair of electrode substrates with a liquid crystal layer being interposed therebetween. No-electrode areas, where no electrode exists, are provided in a discrete pattern partially on at least one of the pair of electrode substrates. A pixel, which is a minimum unit of display, is defined by being surrounded by a plurality of the no-electrode areas. The liquid crystal molecules are oriented in an axially symmetric orientation in each pixel when a voltage is applied between the pair of electrode substrates. The method includes the step of: (e) providing a liquid crystal material into a gap between the pair of electrode substrates.
In one embodiment of the invention, the step (e) includes the step of: (exe2x80x2) attaching the pair of substrates to each other and injecting the prepared precursor mixture into a gap between the pair of substrates. The method further includes the steps of: (a) heating a mixture of a liquid crystal material and a photocurable material to a temperature equal to or greater than a compatibility critical temperature of the mixture, and then cooling the mixture to prepare a precursor mixture; (b) forming electrode lines by pattern etching on one of a pair of substrates: (c) applying a vertical alignment film on each of the pair of substrates; (d) printing a seal material on one of the pair of substrates around a display area; (f) applying an external electric field to tilt liquid crystal molecules by a tilt angle, and polymerizing and curing the photocurable material with the liquid crystal molecules being tilted by the tilt angle, thereby setting an axially symmetric orientation of the liquid crystal molecules based on an orientation memory property of the liquid crystal molecules; and (g) sealing the liquid crystal layer with a sealant.
In one embodiment of the invention, the method further includes the steps of: (b) forming electrode lines by pattern etching on one of a pair of substrates; (c) applying a vertical alignment film on each of the pair of substrates; (d) printing a seal material on one of the pair of substrates around a display area; and (g) sealing the liquid crystal layer with a sealant.
Thus, the invention described herein makes possible the advantage of providing a liquid crystal display device in which the axially symmetric orientation is stabilized and a sufficient response speed can be realized without providing protrusions in a lattice-like pattern.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.